In a gradient amplifier controlled by setpoint or actual value control, the characteristic curve “control voltage at the controller output with respect to output stage output voltage” is intended to be linear, e.g., the output stage output voltage is linearly proportional to the controller voltage. In fact, the characteristic curve is highly nonlinear particularly for low output voltages. The nonlinearity is caused by the forward voltages of the semiconductors (transistors, diodes) in the output stage, the safety times in the driving of the switching transistors, and a “tail current” of the insulated-gate bipolar transducer (IGBT) switching transistors.
The advance control of a differentiating element, configured with the inductance L of the gradient coil, in order to generate a driving in accordance with the formula “output voltage=L*di/dt”, as described in the published patent application DE 198 56 800 A1, functions only if the characteristic curve “(controller+differentiating element)*K=output stage voltage” (K is an element of the real numbers) is linear in accordance with the equation. The nonlinearity present, depending on the current, leads to a deformation of the generated gradient pulses and to a current dependent gradient delay (i.e., a current dependent delay of the gradient pulses). The delay dependent on the gradient amplitude adversely affects the image quality, for example, in radial imaging.
In magnetic resonance imaging apparatuses, about 10% of the performance of the gradient amplifiers suffices for “normal” imaging, such that for “normal” imaging the GPAs of the output stage are constantly operated in the predominantly nonlinear region.
Published patent application WO 2013/046099 A1 proposes a solution of compensation by a three-dimensional lookup table that is loaded from a storage medium. The closed loop control method described is suitable only for digital signal processing as the complicated method does not function rapidly enough in real time. The closed loop control method described uses a controller output signal (1st dimension), an output voltage (2nd dimension), and a filter current (3rd dimension) for determining correction values of a modulation swing.
Closed loop control is a process in which the actual value of a variable is measured and matched to the setpoint value of the variable by readjustment. In open loop control, a machine or installation is influenced with the aid of a manipulated variable, without the controlled variable having a reactive effect on the manipulated variable.